The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling transmission ratio changing speed (rate) at cold engine.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in depending on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced at a speed (rate) which is decided by line pressure, pressure of oil supplied to the servo device of the drive pulley, and actual transmission ratio. In such a system, the speed (rate) of changing of desired transmission ratio (id) is controlled in accordance with driving conditions to eliminate control delay due to the slow response of the transmission control system. In other words, the desired transmission ratio changing speed (rate) did/dt is obtained from a desired drive pulley speed N.sub.p d dependent on load on engine and actual transmission ratio i, desired transmission ratio id dependent on driven pulley speed N.sub.S, and the difference between the actual transmission ratio i and the desired transmission ratio id. Thus, in order to compensate for the control delay, transmission ratio changing speed (rate) di/dt is calculated as follows. EQU di/dt=K1(id-i)+K2.did/dt.
where K1 and K2 are coefficients.
However, the coefficients K1 and K2 are set so as to meet requirements for driving the vehicle when the engine is sufficiently warmed up. Accordingly the coefficients are not appropriate for cold engine, where engine speed does not correspond to an opening degree of a throttle valve. For example, the transmission ratio control system is adapted to downshift the transmission when the vehicle speed does not increase in spite of the increase in the opening degree of the throttle valve. Accordingly, at cold engine when engine speed does not increase even if an accelerator pedal is quickly depressed, the transmission is downshifted, resulting in a decrease of the vehicle speed although acceleration of the vehicle is intended.